Xylose is a valuable raw material in the sweets, aroma and flavoring industries and particularly as a starting material in the production of xylitol. Xylose is formed in the hydrolysis of xylan-containing hemicellulose, for example in the direct acid hydrolysis of biomass, in enzymatic or acid hydrolysis of a prehydrolysate obtained from biomass by prehydrolysis (with steam or acetic acid, for instance), and in sulphite pulping processes. Vegetable materials rich in xylan include the wood material from various wood species, particularly hardwood, such as birch, aspen and beech, various parts of grain (such as straw and husks, particularly corn and barley husks and corn cobs and corn fibers), bagasse, coconut shells, cottonseed skins etc.
Xylose can be recovered by crystallization e.g. from xylose-containing solutions of various origin and purity, such as spent sulphite pulping liquors. In addition to xylose, the spent sulphite pulping liquors contain, as typical components, lignosulphonates, sulphite cooking chemicals, xylonic acid, oligomeric sugars, dimeric sugars and monosaccharides (other than the desired xylose), and carboxylic acids, such as acetic acid, and uronic acids.
Before crystallization, it is as a rule necessary to purify the xylose-containing solution obtained as a result of the hydrolysis of cellulosic material to a required degree of purity by various methods, such as filtration to remove mechanical impurities, ultrafiltration, ion-exchange, decolouring, ion exclusion or chromatography or combinations thereof.
Xylose is produced in large amounts in pulp industry, for example in the sulphite cooking of hardwood material. Chromatographic methods for the separation of xylose from such cooking liquors have been disclosed for example in U.S. Pat. No. 4,631,129 (Suomen Sokeri Oy), U.S. Pat. No. 5,637,225 (Xyrofin Oy) and U.S. Pat. No. 5,730,877 (Xyrofin Oy).
It is also known to use membrane techniques, such as ultrafiltration to purify spent sulphite pulping liquors (e.g. Papermaking Science and Technology, Book 3: Forest Products Chemistry, p. 86, ed. Johan Gullichsen, Hannu Paulapuro and Per Stenius, Helsinki University of Technology, published in cooperation with the Finnish Paper Engineer's Association and TAPPI, Gummerus, Jyväskylä, Finland, 2000). High-molar-mass lignosulphonates can thus be separated by ultrafiltration from the low-molar-mass components, such as xylose.
Nanofiltration is a relatively new pressure-driven membrane filtration process, falling between reverse osmosis and ultrafiltration. Nanofiltration typically retains large and organic molecules with a molar mass greater than 300 g/mol. The most important nanofiltration membranes are composite membranes made by interfacial polymerisation. Polyether sulfone membranes, sulfonated polyether sulfone membranes, polyester membranes, polysulfone membranes, aromatic polyamide membranes, polyvinyl alcohol membranes and polypiperazine membranes are examples of widely used nanofiltration membranes. Inorganic and ceramic membranes can also be used for nanofiltration.
Nanofiltration membranes have been defined by their ability to reject only ions which have a negative charge over one, such as sulphate or phosphate, while passing single-charged ions. The rejection of uncharged, dissolved materials and also of positively charged ions in the solution relate mostly to the size and shape of the molecule in question. The nominal cut-off value of the molecular size relating to nanofiltration is defined to be in the range of 100-1000 g/mol.
Nanofiltration has been used for separating monosaccharides, such as glucose, from disaccharides and higher saccharides. The starting mixture including monosaccharides, disaccharides and higher saccharides may be a starch hydrolysate, for example.
U.S. Pat. No. 5,869,297, Archer Daniels Midland Co. (published Feb. 9, 1999) discloses a nanofiltration process for making dextrose. This process comprises nanofiltering a dextrose composition including as impurities higher saccharides, such as disaccharides and trisaccharides. A dextrose composition having a solids content of at least 99% dextrose is obtained. Crosslinked aromatic polyamide membranes have been used as nanofiltration membranes.
WO 99/28490, Novo Nordisk AS (published Jun. 10, 1999) (=U.S. Pat. No. 6,329,182) discloses a method for enzymatic reaction of saccharides and for nanofiltration of the enzymatically treated saccharide solution including monosaccharides, disaccharides, trisaccharides and higher saccharides. Monosaccharides are obtained in the nanofiltration permeate, while an oligosaccharide syrup containing disaccharides and higher saccharides is obtained in the retentate. The retentate including the disaccharides and higher saccharides is recovered. A thin film composite polysulfone membrane having a cut-off size less than 100 g/mol has been used as the nanofiltration membrane, for example.
U.S. Pat. No. 4,511,654, UOP Inc. (published Apr. 16, 1985) relates to a process for the production of a high glucose or maltose syrup by treating a glucose/maltose-containing feedstock with an enzyme selected from amyloglucosidase and β-amylase to form a partially hydrolyzed reaction mixture, passing the resultant partially hydrolyzed reaction mixture through an ultrafiltration membrane to form a retentate and a permeate, recycling the retentate to the enzyme treatment stage, and recovering the permeate including the high glucose or maltose syrup.
U.S. Pat. No. 6,126,754, Roquette Freres (published Oct. 3, 2000) relates to a process for the manufacture of a starch hydrolysate with a high dextrose content. In this process, a starch milk is subjected to enzymatic treatment to obtain a raw saccharified hydrolysate. The hydrolysate thus obtained is then subjected to nanofiltering to collect as the nanofiltration permeate the desired starch hydrolysate with a high dextrose content.
U.S. Pat. No. 6,406,546 B1, Tate & Lyle Industries (published Jun. 18, 2002) discloses a process of obtaining sucrose from a sucrose-containing syrup by nanofiltering the syrup through a nanofiltration membrane and recovering the nanofiltration retentate enriched in sucrose. It is recited that invert sugars are passed through the nanofiltration membrane into the nanofiltration permeate. FIG. 3 of the publication discloses a three-stage nanofiltration process for obtaining a sucrose-containing nanofiltration retentate. US 2003/0092136A1, D. Delobeau (published May 15, 2003) discloses a process for the manufacture of a starch hydrolysate having a high content of dextrose by a two-stage nanofiltration process. A nanofiltration permeate enriched in dextrose (glucose) is recovered.
US 2002/0079268 A1, J-J Caboche (published Jun. 27, 2002) discloses a process for preparing a fermentation medium for producing high-purity metabolites (such as organic acids, for example optically pure L-lactic acid) from a renewable material (such as wheat solubles or corn steep liquor) by nanofiltration and/or electrodialysis. The purpose of the nanofiltration and/or electrodialysis is to eliminate low molecular weight impurities from the raw material without degrading its concentration of carbon sources.
U.S. Pat. No. 5,965,028, Reilly Industries (published Oct. 12, 1999) discloses a process for the separation of citric acid from less desirable components having a molecular weight similar to that of citric acid (such as glucose and/or fructose) by nanofiltration. A nanofiltration permeate enriched in citric acid is recovered. The feed used for the nanofiltration is typically a clarified citric acid fermentation broth.
M. Saska et al. discuss the decolorization of white cane sugar by nanofiltration in “Direct Production of White Cane Sugar with Clarification and Decolorization Membranes”, Sugar Journal, November 1995, pp. 19 to 21 and December 1995, pp. 29 to 31. Decolorization of ultrafiltered clarified juice was carried out with G-10 thin-film nanofiltration membranes having a molecular weight cut-off of 2500 daltons.
N. Aydogan et al. (Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey) discuss the separation and recovery of sugars by nanofiltration in “Effect of operating parameters on the separation of sugars by nanofiltration”, Separation Science and Technology (1998), 33(12), pp. 1767-1785. For example, it was found that with an increase of the feed flow rate, permeate flux increased. It was also found that there is a linear relationship between the pressure and the permeate flux up to 30 bars. To investigate the effect of the concentration, 1 to 10 weight-% solutions of sucrose and glucose were utilized, whereby it was found that with an increase in the concentration, permeate flux decreased.
M. L. Bruening et al. (Department of Chemistry, Michigan State University, East Lansing, Mich. USA) have investigated the behaviour of multilayer polyelectrolyte membranes in “Nanofiltration with multilayer polyelectrolyte membranes”, PMSE Preprints (2003), 89, 169. It is recited that minimum thickness of the polyelectrolyte films as nanofiltration membranes affords high flux in the nanofiltration. Furthermore, it was found that the charge was the primary factor in the nanofiltration of small neutral molecules (such as methanol and glycerol). It is also recited that the selectivity of 150 between larger neutral molecules (i.e. glucose and sucrose) was achieved.
Chemistry and Industry of Forest Products, vol. 22, No. 1, 2002, pp. 77-81 discloses a review discussing the application of membrane separation in desalinization, concentration and purification of xylan extracts, separation of xylo-oligosaccharides from xylan hydrolysates, and the classification and purification of oligosaccharides. Examples of processing renewable plant resources using membrane separation are given. These include, for example, continuous ethanol fermentation coupled with membrane separation and the concentration of plant xylose solution by nanofiltration.
G. Yang et al. (Membrane Science and Technology Research Center, Nanjing University of Chemical Technology, Nanjing, China) discuss the nanofiltration of xylose in “Concentration of xylose solution through nanofiltration”, Mo Kexue Yu Jishu (2000), 20(5), 21-26 (Journal written in Chinese). In this study, two types of spirally wound nanofiltration modules differing in the cut-off size were used to study the nanofiltration process of crude industrial xylose. It is recited that the xylose solution was concentrated from 4% to 20% by a nanofiltration equipment comprising a 4-stage serial connection configuration.
G. S. Murthy et al. (Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad, India) discuss the concentration of xylose by nanofiltration in “Concentration of xylose reaction liquor by nanofiltration for the production of xylitol sugar alcohol”, Separation and Purification Technology 44 (2005) 221-228. Pilot scale nanofiltration experiments were carried out using a polyamide (PA) spiral membrane module having 300 molecular weight cut-off and 1 m2 effective area. It is recited that at a feed pressure of 20 bar, xylose was concentrated from 2 to 10% at a reasonably high average flux of 241 l/(m2 h) and rejection of >99% which indicated negligible losses of the sugar in the permeate. The feed for the nanofiltration was an acid hydrolysate of rice husk. In accordance with this reference, xylose is concentrated in the nanofiltration retentate. The purity of concentrated xylose product in relation to the other components of the rice husk hydrolysate is not discussed.
Publication CN 1 594 339 A (published 16 Mar. 2005) discloses a method for recovering xylose from waste hydrolyzed fermentation liquor of biomass. The method comprises nanofiltering the liquor at a temperature of 5-50° C., at a pressure of 0.1-3.0 MPa and with a flow rate of 4.0-15.01/min through a nanofiltration membrane to obtain an effluent and then filtering at a temperature of 5-50° C., at a pressure of 0.5-6.0 MPa and with a flow rate of 6.0-20.01/min through a reverse osmosis membrane to obtain recovered concentrated xylose.
WO 02/053783 and WO 02/053781, Danisco Sweeteners Oy (published 11 Jul. 2002) disclose a process of producing a xylose solution from a biomass hydrolysate by subjecting the biomass hydrolysate to nanofiltration and recovering as the nanofiltration permeate a solution enriched in xylose. The feed used for the nanofiltration may be for example a spent sulphite pulping liquor containing a mixture of other closely-related monosaccharides, such as glucose, galactose, rhamnose, arabinose and mannose, in addition to the desired xylose. It was found that the nanofiltration effectively concentrated pentose sugars, such as xylose in the nanofiltration permeate, while hexose sugars retained more in the nanofiltration retentate. However, the permeate obtained from the nanofiltration had a relatively low dry substance content (1 to 2%) and consequently a low xylose content. Furthermore, the xylose yields achieved were low (less than 20%). The xylose flux achieved was less than 0.2 kg xylose m−2 h−1. Hereby the performance of the process was not sufficient for industrial operation .
When the aim 1n industrial nanofiltration processes is to achieve a high xylose yield, it has been as a rule difficult to achieve a high xylose flux (high capacity) at the end of the nanofiltration when approaching the yield value of over 80% and especially over 90%. This concerns both batch processes as well as continuous multistep processes.